Fix issue：（#168 ）MultiSelectComboBox Visible Height

Author: yanjinhuagood

src/WPFDevelopers.Shared/Controls/MultiSelectComboBox/MultiSelectComboBox.cs

@@ -226,7 +226,8 @@ protected override void OnSelectionChanged(SelectionChangedEventArgs e)
                     {
                         if (SelectedItemsExt is IList list)
                         {
-                            list.Clear();
+                            if(list.Count > 0)
+                                list.Clear();
                             foreach (var itm in SelectedItems.Cast<object>())
                                 list.Add(itm);
                         }
@@ -248,7 +249,6 @@ protected override void OnItemsSourceChanged(IEnumerable oldValue, IEnumerable n
                 ApplySearchLogic();
         }
 
-
         public override void OnApplyTemplate()
         {
             base.OnApplyTemplate();
@@ -308,6 +308,20 @@ public override void OnApplyTemplate()
                 _panel = GetTemplateChild(PART_SimpleWrapPanel) as Panel;
             }
         }
+        public MultiSelectComboBox()
+        {
+            IsVisibleChanged += OnMultiSelectComboBox_IsVisibleChanged;
+        }
+
+        private void OnMultiSelectComboBox_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
+        {
+            if (Visibility == Visibility.Visible)
+            {
+                InvalidateMeasure();
+                InvalidateArrange();
+                UpdateLayout();
+            }
+        }
 
         private void ApplySearchLogic()
         {
@@ -525,6 +539,7 @@ private void SearchText(string text)
 
         private void SelectionChecked(ListBox listbox)
         {
+            if (_checkBox == null) return;
             if (listbox.SelectedItems.Count > 0
                 &&
                 listbox.Items.Count == listbox.SelectedItems.Count)